Oscillators are widely used in various electronic devices, e.g., to provide reference clocks, mixing frequencies for telecommunication signals, etc. A negative resistance-based oscillator represents one type of oscillator architecture typically used for the generation of higher frequency signals, such as used in wireless communication devices. Examples of negative resistance-based oscillators include, but are not limited to crystal oscillators, Surface Acoustic Wave (SAW)-based oscillators, etc. Negative resistance-based oscillators comprise an oscillator core having a resonant circuit operatively connected to a negative resistance circuit. The resonant circuit oscillates at the desired resonant frequency, and the negative resistance circuit cancels the resistive losses of the resonant circuit. In effect, the negative resistance circuit eliminates the natural damping of the resonant circuit, and therefore enables the oscillator core to continuously oscillate at the desired resonant frequency.
The successful operation of electronic devices containing such oscillators requires accurate and reliable amplitude control. In particular, amplitude control is necessary due to the fact that different Q-values, e.g., of different resonant circuits, as well as different PVT (Process, Voltage, and Temperature) conditions for any one oscillator may cause wide amplitude variations. For example, an oscillator having a high-Q resonant circuit will have higher amplitude oscillation than an oscillator having a low-Q resonant circuit. Further, an oscillator running in a linear mode requires continuous regulation of the amplitude to prevent the oscillator amplitude from quickly falling to zero or increasing to a level limited by the nonlinear effects, e.g., voltage clipping, of the oscillator. Such voltage clipping can greatly deteriorate oscillator performance, increase the risk of parasitic oscillation, increase the current consumption (depending on circuit topology), and generally make the behavior of the oscillator more unpredictable. Accurate and reliable amplitude control will equalize the amplitude variations across a wide range of Q-values and PVT conditions, as well as ensure good noise performance, provide low current consumption, avoid parasitic oscillation, and possibly prevent damage to active and passive components
A negative feedback loop provides one way to control the amplitude of the oscillator output, where the negative feedback loop senses the amplitude of the oscillator output and then adjusts the amplitude by controlling an operating point of the oscillator core. For example, controlling the current through active transistor devices of the oscillator core controls the transconductance gm of the oscillator core to control the negative resistance, and thus controls the oscillator amplitude. However, such negative feedback loops may introduce noise into the oscillator core, particularly when the negative feedback loop has a high gain. Further, the nonlinear properties of the oscillator core will convert the input noise to both AM (Amplitude Modulation) and PM (Phase Modulation) noise. While increasing the loop gain of the negative feedback loop will reduce the AM noise, such an increased loop gain will not only increase the power consumption, but will also fail to reduce the PM noise. While reducing the bandwidth of the negative feedback loop will also reduce the noise, such a bandwidth reduction, however, will increase the startup time of the oscillator, and may also undesirably increase the size (consumed chip area) of any filter required to filter the oscillator input signal. Thus, such bandwidth reduction is also not desirable.
As noted above, negative resistance-based oscillators are particularly useful for high frequency applications, and may be particularly important for mmW (millimeter wave) communication. Also, specifically for reference oscillators based on e.g., crystal or SAW resonators, the use of even higher frequencies is anticipated, from todays 10's of MHz to 100's of MHz and possibly even frequencies approaching the GHz range. The generation of such higher frequencies generally results in higher power consumption. Further, the generation of such higher frequencies also presents design challenges due to increased tolerances of the resonators, increased noise, increased component sizes, longer startup times, and/or larger impacts from parasitic elements of the circuitry and associated package. Thus, there remains a need for improved higher frequency generation circuits that do not incur higher power consumption, higher noise, and/or longer start-up times.